The present invention relates to an improved process for removal of metal ions from aqueous solutions. In particular the present invention relates to the use of polyamines and cation exchange resins to remove metal ions from metal-plating and metal-finishing waste streams.
In the process of electroless metal plating it is desirable to maintain a high concentration of the metal ion in solution for plating onto a substrate. This is achieved by solubilizing normally insoluble metal ions, such as copper and nickel, as their metal complexes under neutral or basic conditions.
It has been recognized in electroless metal plating processes that using an excess of a chelating or complexing agent, such as ethylenediaminetetraacetic acid (EDTA), maintains the solubility of the metal ions so that undesirable deposition and precipitation do not occur during the process. However, removal of low-level concentrations of the metal ions from process waste streams is rendered difficult and often impractical due to the stability of the metal complexes formed during the plating process. Anion exchange resins have been used to adsorb the negatively charged metal ion complexes from used plating solutions; however, the background concentration of excess competing anions, such as sulfate, and excess complexing agent, are usually more readily adsorbed by the anion exchange resins and the effective capacity of such resins for the metal ion complex is relatively low causing incomplete removal of metal ions from these solutions.
Low-level concentrations of residual metal ions in waste streams from metal-finishing and electroplating operations pose a significant environmental problem due to the toxic nature of many of the metal ions if they contaminate groundwater systems that are used by wildlife or human populations. It is, therefore, desirable to minimize contamination of water systems by metal ions, and there is a need for an economical and efficient method for removal of metal ions from waste streams containing low-level metal concentrations.
A variety of methods have been employed to address the problem of removing metal ions from metal-process waste streams. For example, U.S. Pat. No. 4,167,481 discloses a process for removing metal ions from aqueous solutions by using polyamines in amounts greater than the stoichiometric amount of metal ion in the waste streams to enhance the adsorption of the metal ions onto cation exchange materials. However, the high use levels required for the polyamines themselves present additional disposal problems.
The methods of the prior art suffer from incomplete removal of metal ions using anion exchange resins or require high levels of polyamine additives to increase the efficiency of cation exchange materials for removing metal ions. These approaches increase the cost and complexity of treating the waste streams from metal-working and metal-plating processes and may require additional treatment. The present invention seeks to overcome the problems of the prior art by providing an improved process for removing metal ions from aqueous solutions.